A block copolymer comprising a vinyl aromatic hydrocarbon and a conjugated diene, which has a relatively high vinyl aromatic hydrocarbon content, has been used for injection molding applications and extrusion molding applications for sheets, films and the like, utilizing characteristics such as transparency and impact resistance. In particular, a heat shrinkable film using the block copolymer comprising a vinyl aromatic hydrocarbon and a conjugated diene has no problems of a residual monomer, a residual plasticizer and generation of hydrogen chloride in burning of a vinyl chloride resin which has hitherto been used, so that it has been utilized for food packaging, cap seals, labels and the like. As characteristics necessary for a heat shrinkable film, there are requirements such as natural shrinkability, low-temperature shrinkability, transparency, mechanical strength and aptitude for packaging machinery. In order to improve these characteristics and obtain a good balance of physical properties, various studies have been made.
Patent document 1 specified below discloses a composition of a vinyl aromatic hydrocarbon-aliphatic unsaturated carboxylic acid-based derivative copolymer having an aliphatic unsaturated carboxylic acid-based derivative content of 5 to 80% by weight and a Vicat softening point not exceeding 90° C., and a copolymer comprising blocks of a vinyl aromatic hydrocarbon and a conjugated diene, in order to obtain a composition excellent in mechanical characteristics, optical characteristics, stretching characteristics, crack resistance characteristics and the like.
Patent document 2 specified below discloses a heat shrinkable film which has a specific glass transition point (Tg) in segments of a block copolymer comprising a vinyl aromatic hydrocarbon and a conjugated diene, in order to obtain a heat shrinkable film excellent in shrinkable characteristics and environmental destruction resistance.
Patent document 3 specified below discloses a heat shrinkable film which comprises a composition of a block copolymer comprising a vinyl aromatic hydrocarbon having a specific structure and a conjugated diene, in order to obtain a heat shrinkable film excellent in shrinkable characteristics and environmental destruction resistance.
Patent document 4 specified below discloses a low-temperature shrinkable film obtained by stretching a composition of a vinyl aromatic hydrocarbon-aliphatic unsaturated carboxylic acid-based derivative copolymer having a vinyl aromatic hydrocarbon content of 95 to 20% by weight and a Vicat softening point not exceeding 90° C. and a copolymer comprising blocks of a vinyl aromatic hydrocarbon and a conjugated diene, in order to obtain a shrinkable film excellent in low-temperature shrinkability, optical characteristics, crack resistance characteristics, dimensional stability and the like.
Patent document 5 specified below discloses a polystyrene-based heat shrinkable film comprising a composition of a block copolymer comprising a styrene-based hydrocarbon and a conjugated diene hydrocarbon and a styrene-based hydrocarbon-containing random copolymer having a specific Tg, in order to improve natural shrinkability at room temperature.
Patent document 6 specified below discloses a heat shrinkable hard film which comprises a composition of a vinyl aromatic hydrocarbon-aliphatic unsaturated carboxylic acid-based derivative copolymer having a Vicat softening point not exceeding 105° C. and a copolymer comprising blocks of a vinyl aromatic hydrocarbon and a conjugated diene, and has a specific heat shrinkage force, in order to obtain a transparent heat shrinkable film excellent in stability with time and impact resistance of the film.
Patent document 7 specified below discloses a composition of a copolymer comprising blocks of a vinyl aromatic hydrocarbon having a specific structure and molecular weight distribution and a conjugated diene and a vinyl aromatic hydrocarbon-(meth)acrylic ester copolymer resin, in order to obtain a composition balanced in transparency, rigidity and low-temperature surface impact properties.
Patent document 8 specified below discloses a transparent high-strength resin composition containing a block copolymer comprising a vinyl aromatic hydrocarbon block having a specific structure and a copolymer block of a vinyl aromatic hydrocarbon and a conjugated diene, and a copolymer of a vinyl aromatic hydrocarbon and a (meth)acrylic ester, in order to obtain a resin composition excellent in transparency and impact resistance.
Patent document 9 specified below discloses a multilayer low-temperature shrinkable film having at least one layer of a composition of a vinyl aromatic hydrocarbon-aliphatic unsaturated carboxylic acid-based derivative copolymer having a vinyl aromatic hydrocarbon content of 95 to 20% by weight and a Vicat softening point not exceeding 90° C. and a copolymer comprising blocks of a vinyl aromatic hydrocarbon and a conjugated diene, in order to obtain a shrinkable film excellent in low-temperature shrinkability, optical characteristics, crack resistance characteristics, dimensional stability and the like.
Patent document 10 specified below discloses a multilayer polystyrene-based heat shrinkable film which has at least three layers having both outer layers comprising a mixture of a styrene-butadiene-styrene type block copolymer having a specific butadiene unit content and styrene-butyl acrylate and an intermediate layer comprising a mixture of a styrene-butadiene-styrene type block copolymer having a specific butadiene unit content and styrene-butyl acrylate, in order to obtain a shrinkable film excellent in natural shrinkability, strength, surface characteristics, resiliency, low-temperature shrinkability and the like.
Patent document 11 specified below discloses a heat shrinkable polystyrene-based laminated film comprising as an intermediate layer a compound in which a block copolymer comprising a vinyl aromatic hydrocarbon and a conjugated diene-based hydrocarbon is combined with a copolymer of a vinyl aromatic hydrocarbon and an aliphatic unsaturated carboxylic ester, and as front and back layers a mixed copolymer mainly composed of a block copolymer comprising a vinyl aromatic hydrocarbon and a conjugated diene-based hydrocarbon, in order to obtain a heat shrinkable film excellent in any characteristic of natural shrinkability, resistance to heat fusion bonding, transparency and shrinkage finishing properties.
Patent document 12 specified below discloses a multilayer heat shrinkable polystyrene-based film having a specific degree of heat shrinkage, in which an intermediate layer is mainly composed of a styrene-(meth)acrylic ester copolymer having a specific Vicat softening point, and inner and outer layers are mainly composed of a styrene-conjugated diene block copolymer having a specific Vicat softening point, in order to obtain a heat shrinkable film excellent in heat shrinkability at low temperatures, shrinkage finishing properties and the degree of natural shrinkage, and undergoing no blocking with each other in a heated state.
Patent document 13 specified below discloses a (multilayer) heat shrinkable film having a layer mainly composed of a copolymer resin comprising a styrene-based monomer and a (meth)acrylic ester-based monomer, which is characterized by a specific molecular weight distribution and residual monomer amount, a block copolymer resin comprising styrene and a conjugated diene, and a high-impact polystyrene resin composition, in order to obtain a resin composition, a film and a multilayer film which are excellent in processing characteristics and storage stability, reduced in odor, and excellent in rigidity and impact resistance.
Patent document 14 specified below discloses a hydrogenated copolymer characterized by a specific weight ratio of a conjugated diene monomer and a vinyl aromatic hydrocarbon monomer, an amount of polymer blocks of a vinyl aromatic hydrocarbon monomer, and absence of crystallization in a DSC chart within a specific temperature range, in order to obtain a hydrogenated copolymer rich in flexibility, excellent in impact resilience and scratch resistance, and good in handling properties (blocking resistance).
Patent document 15 specified below discloses a hydrogenated copolymer having a specific structure, in order to obtain a composition good in puncture impact value, strength, transparency and the like.
However, these block copolymers comprising a vinyl aromatic hydrocarbon and a conjugated diene, or the compositions comprising the block copolymer and a vinyl aromatic hydrocarbon-aliphatic unsaturated carboxylic acid-based derivative copolymer, or the hydrogenated copolymers comprising a vinyl aromatic hydrocarbon and a conjugated diene and the compositions thereof are insufficient in a balance of physical properties such as solvent resistance, natural shrinkability, low-temperature shrinkability, rigidity, transparency and impact resistance suitable for a sheet and a heat shrinkable film which are excellent in impact resistance, solvent resistance, rigidity and transparency. These documents do not disclose any methods for improving them, and the problems in the market have still been pointed out.
Patent Document 1: JP-A-59-221348
Patent Document 2: JP-A-60-224520
Patent Document 3: JP-A-60-224522
Patent Document 4: JP-A-61-25819
Patent Document 5: JP-A-4-52129
Patent Document 6: JP-A-5-104630
Patent Document 7: JP-A-6-220278
Patent Document 8: JP-A-7-216187
Patent Document 9: JP-A-61-41544
Patent Document 10: JP-A-2000-185373
Patent Document 11: JP-A-2000-6329
Patent Document 12: JP-A-2002-46231
Patent Document 13: JP-A-2002-201324
Patent Document 14: WO 03/035705
Patent Document 15: WO 03/066697